Witch or magician level 12. A willing partner of the other class and identical level.
You can never attain more than one level more than your bondmate, and can only ever be part of one spirit bond at any time.

Prerequisite for witch lady and mage lord prestige classes. Note that this bond, particularly the locate aspect, enables a witch lady to act as an anchor for her bondmate when he travels the planes, ensuring that he can always find his way home.